Life-time studies with capillary electrochromatography columns operated under different conditions

A test system has been established to permit the monitoring of the life-time performance of several reversed- phase capillary electrochromatography (CEC) columns. The retention factors, kcec, peak symmetry coefficients, λsym, and column efficiencies, N, of three neutral n-alkylbenzene analytes, namely ethyl-, n-butyl- and n-pentylbenzenes, were determined for Hypersil 3 μm n-octylsilica and n-octadecylsilica packed into CEC capillary columns of 100 μm I.D., with a packed length of 250 mm, and a total length of 335 mm. The performances of these CEC capillary columns were examined for a variety of eluents with pH values ranging between pH 2.0 – 8.0, similar to those employed to study the retention behaviour of peptides that we have previously reported. The relative standard deviation (RSD) of the retention factors (kcec values) of these n-alkylbenzenes, acquired with an eluent of (25 mM Tris–HCl, pH 8.0,)–acetonitrile (1:4, v/v), when the CEC capillary columns were used for the first time (virgin values), were 4% (based on data acquired with 4 CEC capillary columns) for the n-octyl bonded silica capillary columns, and 6% (based on 8 columns) for n-octadecyl bonded silica capillary columns. The RSD values of the kcec values of the n-alkylbenzenes for one set of replicates (n=6) with one CEC capillary column was <0.5%. The theoretical plate numbers, N, for the virgin CEC capillary columns were ca. 60 000, whilst the observed N values for all new CEC capillary columns were ≥40 000 for n-octyl bonded silica capillary columns and ≥50 000 for n-octadecyl bonded silica capillary columns. The peak symmetry coefficients, λsym, of the n-alkylbenzenes for virgin CEC capillary columns and for CEC capillary columns used for more than 1000 injections were always in the range 0.95–1.05. The experimental results clearly document that the life-time performance of the CEC capillary columns depends on the eluent composition, as well as the nature of the analytes to which the CEC capillary columns are exposed.